a) Field of the Invention
The present invention relates to a solid-state image pickup device used as an area image sensor and a driving method of the same, and in particular, to a solid-state image pickup device of interline transfer type comprising a plurality of photoelectric converter columns and the same number of vertical transfer charge-coupled devices (CCD) as the photoelectric converter columns and a driving method of the same.
b) Description of the Related Art
Development of mass-production technology of CCDs enhances a quick spread of a video camera, an electronic still camera, and the like using a solid-state image pickup device of CCD type as an area image sensor. Solid-state image pickup devices of CCD type are classified into several kinds according to structure including a solid-state image pickup device of interline transfer type (to be referred to as “IT-CCD” herebelow).
An IT-CCD comprises a large number of photoelectric converters or photoelectric converter elements arranged on a surface of a semiconductor substrate in a plurality of columns with a constant pitch and in a plurality of rows with a constant pitch. Each of the columns and the rows includes a plurality of photoelectric converters. Each photoelectric converter is comprised of a photo diode in ordinary cases.
For example, to form a large number of photoelectric converters being composed of p-n photodiodes, a p-type well is manufactured on a desired surface of a semiconductor substrate and n-type regions each having a desired contour are formed in the p-type well as many as there are photoelectric converters. If necessary, a p+-region is manufactured on each n-type region. Signal charge is stored in each n-type region. Namely, each n-type region serves as a signal charge storage region.
In this specification, a term “photoelectric converter (element)” indicates only a signal charge storage region depending on cases. Moreover, “adjacent to a photoelectric converter” means that “adjacent to a signal charge storage region constituting a photoelectric converter”and “contiguous to a photoelectric converter” indicates “contiguous to a signal charge storage region constituting a photoelectric converter”.
In the IT-CCD, for each photoelectric converter column, one charge transfer channel is formed adjacent thereto. Therefore, an IT-CCD includes a plurality of charge transfer channels. Each charge transfer channel is used to transfer signal charge stored in respective photoelectric converters of each photoelectric converter column adjacent to the charge transfer channel.
A plurality of transfer electrodes are formed on the surface of the semiconductor substrate with an electrically insulating film intervening therebetween, the transfer electrodes intersecting the charge transfer channels in plan view. Each intersection between the electrodes and the channels serves as one charge transfer stage. That is, one vertical transfer CCD includes one charge transfer channel and the transfer electrodes.
In this specification, a region of the charge transfer stage in each transfer electrode of the vertical transfer CCD is called “transfer path forming region”.
Each vertical transfer CCD of the IT-CCD of interlace driving type ordinarily has two charge transfer stages for one photoelectric converter. Each vertical transfer CCD of the IT-CCD of all pixel readout type ordinarily includes three or four charge transfer stages for each photoelectric converter.
Each photoelectric converter stores signal charge by achieving photoelectric conversion. The signal charge is read therefrom to an associated charge transfer channel at a predetermined point of time.
To control the operation to read of signal charge from the photoelectric converters to the charge transfer channels, one readout gate region is disposed for each photoelectric converter to be contiguous to the photoelectric converter. Each readout gate region is formed on the surface of the semiconductor substrate to be contiguous to an associated photoelectric converter and to a charge transfer channel corresponding to the photoelectric converter. Ordinarily, to form a potential barrier against the signal charge, the readout gate region has a conductivity type opposite to that of the photoelectric converter and the charge transfer channel.
Each readout gate region has a readout gate electrode region thereon. Each readout gate electrode region is ordinarily part of the transfer path forming region of a predetermined transfer electrode of the vertical transfer CCD. By applying a high voltage to the readout gate electrode region to remove or to lower the potential barrier in the readout gate region, signal charge accumulated in the photoelectric converter can be read out to the charge transfer channel.
Signal charge on each charge transfer channel is transferred to an output transfer path by each vertical transfer CCD comprising the charge transfer channel. The output transfer path is ordinarily composed of CCDs (which are referred to as “horizontal transfer CCD in some cases).
The output transfer path being composed of horizontal transfer CCD includes N charge transfer stages for one vertical transfer CCD. Ordinarily, one charge transfer stage includes one potential barrier region and one potential well region and N is two. When each charge transfer stage has substantially uniform potential, N is three or more.
The output transfer path sequentially transfers received signal charge in a longitudinal direction (to be referred to as “row direction” herebelow) of the photoelectric converter row. This resultantly outputs received signal charge to an output portion. Like the vertical transfer CCD, the output transfer path is formed also on the semiconductor substrate.
Each of the vertical transfer CCD and the horizontal transfer CCD has a function of photoelectric conversion. Therefore, to prevent unnecessary photoelectric conversion in the vertical and horizontal CCDs, a light shielding film is arranged. The film has an opening with a predetermined contour on each photoelectric converter (photodiode). One opening is disposed for each photoelectric converter. The opening is ordinarily disposed in plan view within an area defined by a peripheral edge of a signal charge storage region of the photoelectric converter.
One pixel includes one photoelectric converter, one readout gate region formed contiguous to the photoelectric converter, one readout gate electrode region over the readout gate region in plan view, and two to four charge transfer stages (of the vertical transfer CCD) corresponding to the photoelectric converter. On the surface of each photoelectric converter, an area exposed by the opening in plan view serve as a light receiving section of the pertinent pixel.
In an IT-CCD, the contour and area of the light receiving section are therefore substantially determined by the contour and the area of each opening formed in the light shielding film in plan view.
Broad development of the IT-CCD requires further improvement of performance of the IT-CCD in, for example, resolution and sensitivity.
The resolution of the IT-CCD considerably depends of a pixel density (a degree of integration). The more the pixel density (degree of integration) is, the easier the improvement of resolution is. On the other hand, the sensitivity greatly depends on the area of the light receiving section of each pixel. The larger the area is, the easier the improvement of the sensitivity is.
Japanese Patent Publication Ser. No. 2825702 describes an IT-CCD (the device is referred to as “solid-state image pickup device” therein, but will be referred to as “IT-CCD” in this specification) in which the pixel density can be increased while suppressing the decrease in the area of the light receiving section of each pixel.
This IT-CCD comprises a large number of photoelectric converters in a plurality of columns with a constant pitch and in a plurality of rows with a constant pitch. Each of the columns and the rows includes a plurality of photoelectric converters. The photoelectric converters in even columns are respectively shifted in a direction of the column from those in odd columns about one half of the pitch of photoelectric converters in each photoelectric converter column. Similarly, the photoelectric converters in even rows are respectively shifted in a direction of the row from those in odd row about one half of the pitch of photoelectric converters in each photoelectric converter row. Each photoelectric converter column is composed of photoelectric converters of only odd or even rows.
To transfer signal charge stored in each photoelectric converter, a plurality of vertical transfer CCDs are arranged. Each vertical transfer CCD, locally meandering, transfers the signal charge in a predetermined direction.
Each vertical transfer CCD comprises a plurality of transfer electrodes arranged in a honeycomb layout. Arrangement of these transfer electrodes in such a shape configures gaps or regions each having a hexagonal contour. In each of the hexagonal regions, one photoelectric converter exists in plan view.
Signal charge stored in each photoelectric converter in an odd column is read out to an associated vertical transfer CCD via a readout gate region formed in plan view adjacent to an inner side of a lower-left edge of the hexagonal region. On the other hand, signal charge stored in each photoelectric converter in an even column is read out to the associated vertical transfer CCD via a readout gate region formed in plan view adjacent to an inner side of an upper-left edge of the hexagonal region.
In the IT-CCD described in the publication, by arranging a large number of photoelectric converters and a plurality of transfer electrodes (for the vertical transfer CCD) as above, the pixel density can be increased while suppressing the decrease in the area of the light receiving section of each pixel.
In this specification, the arrangement of a large number of photoelectric converters above is referred to as “shifted-pixel layout”.
In the IT-CCD, for each photoelectric converter, one microlens is formed thereover in ordinary cases. Light from an object is collected by an optical system of an image-pickup or imaging lens. Resultant light is further collected by the microlens to produce an image on the photoelectric converter.
In this situation, a light beam incident to the microlens at a position in an upper section of a photoelectric converter column has an incidence angle which is reverse, with respect to an optical axis of the image-pickup lens, to an incidence angle of a light beam incident to the microlens in a lower section of the photoelectric converter column. Therefore, between the upper and lower sections of the photoelectric converter column, a position of the image produced by the microlens on the photoelectric converter is also reversed with respect to the optical axis of the image-pickup lens.
FIG. 34 is a cross-sectional view for explaining a position of an image formed on a photoelectric converter by the microlens. A photoelectric converter 401 shown in FIG. 34 is formed on a semiconductor substrate 402. A microlens 403 is formed over the converter 401 with a focus adjusting layer 404 intervening therebetween. In FIG. 34, a photoelectric converter column extends in right and left direction.
When the photoelectric converter 401 exists in an upper section of the photoelectric converter column, a light beam 405 which cross an optical axis 403a of the microlens 403 along an inclined direction from a central side of the photoelectric converter column to the upper section of the photoelectric converter column enters the microlens 403. The light beam 405 produces an image focused at a point 405a shifted upward (on an upper side in the direction of the photoelectric converter column) with respect to the optical axis 403a of the microlens 403.
On the other hand, when the photoelectric converter 401 exists in a lower section of the photoelectric converter column, a light beam 406 which cross the optical axis 403a of the microlens 403 along an inclined direction from a central side of the photoelectric converter column to the lower section of the photoelectric converter column enters the microlens 403. The light beam 406 produces an image focused at a point 406b shifted downward (on a lower side in the direction of the photoelectric converter column) with respect to the optical axis 403a of the microlens 403.
The amount of shift of the point 405a from the optical axis 403a of the microlens 403 increases as the position of the microlens 403 becomes farther from the central position of the photoelectric converter column. This also applies to the amount of shift of the point 406b relative to the optical axis 403a of the microlens 403.
Also in the IT-CCD in which pixels are disposed in a quadratic or square lattice format and the IT-CCD in which pixels are arranged in a shifted-pixel layout, light collecting efficiency and sensitivity of each pixel vary between two adjacent pixel rows in cases (A) to (C) as follows.    (A) The light receiving section of each pixel has a different contour.    (B) The contour of the light receiving section is almost uniform in each pixel, but the sections vary in size thereof.    (C) The contour and the size of the light receiving section are almost uniform in each pixel, but the light receiving sections vary in direction thereof.
When light collecting efficiency and sensitivity of each pixel vary between two adjacent pixel rows, for example, in an IT-CCD for color images, signals produced from the IT-CCD become deteriorated in color balance and hence an image produced is attended with color shading. In an IT-CCD for monochrome images, an image reproduced is lowered in its quality by unevenness in a background of the image.
For example, in the IT-CCD of Japanese Patent Publication Ser. No. 2825702, the hexagonal gaps or regions appearing in the honeycomb arrangement of a plurality of transfer electrodes have a shift of 180° in plan view between the regions in odd rows and those in even rows. Therefore, in this IT-CCD, when the contour and the size of the light receiving section of each pixel is fabricated in a form analogous to a form obtained by directly minimizing the hexagonal region, the light collecting efficiency and sensitivity of each pixel easily vary between two adjacent pixel rows. Resultantly, color shading and background unevenness easily take place in the IT-CCD.
In this description, “regions in odd rows” and “regions in even rows” respectively mean “regions in odd rows” and “regions in even rows” when the regions are arranged in an array in which the longitudinal direction (to be referred to as “column direction” herebelow) of the photoelectric converter column and that of the photoelectric converter row are assumed to be the column direction and the row direction, respectively.
In the IT-CCD of Japanese Patent Publication Ser. No. 2825702, when the light receiving section of each pixel between two adjacent pixel rows is fabricated to have an almost identical contour and size and an almost identical direction, the area of the light receiving section becomes further narrower than the hexagonal region. That is, an area available for the light receiving section is decreased. It is therefore difficult to increase the pixel density while preventing the minimization of the area of the light receiving section of each pixel.
Heretofore, in the charge transfer channel of the vertical transfer CCD, a section thereof contiguous to the readout gate region and a section thereof separated from the readout gate region have substantially equal channel width.
Consequently, the transfer path forming region including the readout gate electrode region must be have a width larger than that not including the readout gate electrode region. Alternatively, each transfer path forming region is formed to have a width substantially equal to the width of the transfer path forming region including the readout gate electrode region.
For example, in the IT-CCD of the Patent Publication above, when the width of the transfer path forming region becomes greater, the area of the hexagonal region appearing in the honeycomb arrangement of the transfer electrodes of the vertical transfer CCD is minimized in plan view. Since the photoelectric converter is manufactured in the hexagonal region as viewed in plan, the area of the photoelectric converter is also decreased as that of the hexagonal region is minimized in plan view.
As described above, the contour of the light receiving section of each pixel is naturally determined by that of the openings in plan view formed in the light shielding film. However, the opening is ordinarily manufactured inside the peripheral edge of the associated photoelectric converter in plan view. This means that the area of the light receiving section of the pixel cannot be larger than that of the photoelectric converter of the pertinent pixel.
Therefore, when the area of the photoelectric converter becomes smaller, the area of the light receiving section of the associated pixel is also decreased in ordinary cases.